The genetic resistance to Friend virus induced leukemia is mediated, to a great extent, by marrow-dependent cells depleted by treatment of mice with 89Sr. One important marrow dependent function is to regulate T and non-T suppressor cells in the body. Estradiol also depletes marrow dependent cells but also depletes suppressor cells. Therefore, we will test the importance of suppressor cells by comparing the effects of 89Sr and estradiol on the susceptibility of mice to Friend virus induced leukemia and immunosuppression. Assays will include natural killer cells for FLD-3, an Friend virus induced erythroleukemia cell line, humoral antibody formation and cell-mediated lympholysis. Genetically susceptible mice will be infected with Friend virus to induce leukemia and will then be treated with total-body irradiation followed by reconstitution with marrow cells from genetically ressistant mice. The donors will be H-2 allogeneic or H-2 identical allogeneic and the marrow cells will be treated with anti-T cell antibodies to prevent graft-versus-host disease. Survival of the mice will be determined, as well as virological studies.